Plant parasitic nematodes cause serious economic damage to many agricultural crops around the world. This group of nematodes are microscopic worms and are, in general, obligate parasites of plants. They feed mostly on the roots of host plants; however, several genera are known to parasitize above ground parts including stem, leaves and flowers as well. Almost all the major plant species are susceptible to infection by species of nematodes (notable exceptions are in the marigolds and asparagus). For example, one of the most important genera of plant nematodes, root knot nematodes, (Meloidogyne spp.) is capable of parsitising more than 3,000 species of crop plants. These plants include agronomic crops, vegetables, fruits, flowering trees and shrubs. Nematodes reportedly cause crop loss equivalent to more than U.S. $6 billion in the United States alone and more than U.S. $100 billion around the world.
The symptoms due to phytoparasitic nematode injury varies widely depending on the plant host (even variety), the nematode species (even race), age of the plant, geographical location, climatic conditions etc. In general, an overall patchy appearance of plants in a field is considered indicative of nematode infestation. More specifically, nematode injury results in galling of the roots (abnormal swelling in the tissues due to rapid multiplication of cells in the cortical region) caused by species of root knot (Meloidogyne spp.) and cyst (Heterodera spp.) nematodes, lesions (localized, discolored area) caused by lesion nematodes (Pratylenchus spp.), suppression of cell division resulting in `stubby` roots (Trichodorus spp.), growth abnormalities including crinkling or twisting of above-ground parts (Aphelenchoides spp.) and even cell necrosis (death) in some cases. Plant parasitic nematodes may be endoparasitic in nature as in the case of the root-knot and lesion nematodes or ectoparasitic as in the dagger nematode (Xiphinema spp.) and lance nematode (Hoplolaimus spp.). Nematodes can be vectors of plant viruses and are also known to induce disease complexes by creating infection courts for the entry of other plant pathogenic fungi and bacteria.
Chemical nematocides as soil fumigants or non-fumigants have been in use for many years and represent one of the few feasible processes for countering nematodes. At present, the process involves repeated applications of synthetic chemicals to the ground prior to planting the crop. These chemicals are extremely toxic to organisms besides nematodes and pose serious threat to the environment. With the renewed emphasis on clean water and air by the United States Environmental Protection Agency, and the detection of many of these active ingredients or the metabolites thereof in ground water and in several non-target organisms, there has been serious concern as to the manufacture and/or use of these chemicals. One of the most effective, economical, and widely used nematocides, DBCP (1,2-dibromo-3-chloropropane) was judged to incite male sterility and possible carcinogenesis an was reported in ground water. Another widely used chemical, EDB (ethylene dibromide), was also found in ground water. Yet another very common insecticide-nematocide, aldicarb (2-methyl-2-(methylthio) propionaldehyde O-(methylcarbmoyl)oxime), was found to be acutely toxic and was found in ground water in several regions of the United States. Carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarabmate) and 1,3-D (1,3-dichloropropane), two very commonly used nematocides, are under special review by the EPA, because of their avian toxicity and possible carcinogenic effects.
No known commercially acceptable biological agents have been effective in controlling nematodes to date.